The deployment of Session Initiation Protocol (SIP) in enterprise networks gives users significantly greater flexibility to use and control commonly used communication technologies. SIP is an application-layer control or signaling protocol used for creating, modifying and terminating sessions that contain one or more participants. These sessions include Internet telephone calls, for example Voice over Internet Protocol (VoIP), voice calls, video calls, multimedia distribution, multimedia conferences, Instant Messaging (IM) and presence detection. An example of the use of SIP for presence detection is found in A. Niemi et al., Session Initiation Protocol (SIP) Extension for Presence Publication, SIMPLE Working Group Internet-Draft, June 2003.
The application of SIP can be for point-to-point or multiparty sessions. SIP invitations used to create sessions carry session descriptions that allow participants to agree on a set of compatible media types. SIP makes use of elements called proxy servers to help route requests to each user's current location, authenticate and authorize users for services, implement provider call-routing policies, and provide features to users. SIP also provides a registration function that allows users to upload their current locations for use by proxy servers. A description of SIP can be found in J. Rosenberg et al., SIP: Session Initiation Protocol, RFC 3261, IETF, June 2002.
In general, SIP runs on top of different transport protocols and is independent of the media transport, which, for example, typically uses Real-Time Transport Protocol (RTP) over User Datagram Protocol (UDP). An example of RTP can be found in H. Schulzrinne et al., RTP: A Transport Protocol for Real-Time Applications, RFC 1889, IETF, January 1996. SIP allows multiple end-points to establish media sessions with each other by supporting locating the end-points, establishing the session and then, after the media session has been completed, terminating the session. Recently, SIP has gained widespread acceptance and deployment among wireline service providers for introducing new services such as VoIP, within the enterprise for Instant Messaging and collaboration, and for push-to-talk service amongst mobile carriers.